Mechanical overloading of hoists occurs frequently in their regular everyday operation. Often, frequent overloading is overlooked in an effort to avoid operation and production delays. Nevertheless, mechanical overloading is undesirable whether it occurs inadvertently or as a routine, intentional practice. Hoists and cranes incorporating hoists typically have mechanical overload monitoring devices which sense mechanical strain on the hoist. In these devices, a strain sensor is placed in engagement with the hoist apparatus at a location which may, for example, be the most critical mechanical stress area of the hoist. An output signal from the strain sensor indicative of an overload is fed to the hoist control and used to provide an overload indication and possibly prevent further operation of the hoist. Typically, several relays and their contacts are operated in order to utilize the output signal from the strain sensor. An example of one type of mechanical strain overload monitoring device has a load cell located in contact with the dead end of the wire rope of the hoist at the location of the rope attachment to the hoist frame. The load cell produces an output signal indicative of the strain produced in the rope by the weight of the load object raised and carried or lowered by the hoist.
Alternating current adjustable frequency drives for various types of motor applications have been in use for some time. More recently, adjustable frequency motor drives have been developed for hoist applications, however, due to exacting load control requirements required by hoists, various problems with adjustable frequency drives have prevented their wide spread use. One problem with adjustable frequency drives has been their lack of reliability in producing the necessary torque to control the load at the initiation of hoist operation. A recently developed solution to this problem involves the maintaining of the hoist brake engaged at the beginning of motor operation, applying power from the adjustable frequency drive at a low frequency only sufficient to produce the current and thereby the torque necessary to control the load on the hoist when the brake is released, and releasing the brake only after the necessary current level is detected.
The above described approach has made adjustable frequency drive systems far more reliable in their load controlling ability at the initiation of hoist operation, however, further development of adjustable frequency drive systems to provide them with additional characteristics and reliability is desirable to make them competitive with other types of drive systems. One of these desirable characteristics is the ability to detect mechanical overloading of the hoist.